The capacitive touch panels which detect the position of an input provided by an input operation object such as a finger are classified into a self-capacitance scheme (one-wire type) and a mutual capacitance scheme (two-wire type). The self-capacitance scheme is configured to detect a detection electrode that is increased in stray capacitance by an input operation object approaching the same and thereby detect the position of an input operation from the position at which the detection electrode is disposed. The mutual capacitance scheme is configured to apply a detection voltage at a predetermined voltage level to a drive electrode and detect a detection electrode of which detection voltage level is reduced by an input operation object approaching the same so as to detect the position of an input operation from the position at which the detection electrode is disposed. The former scheme provides a simplified structure because no drive electrode is employed, but detects stray capacitance at an infinitesimal level of 10 to 20 pF with difficulty. Thus, in general, the latter mutual capacitance scheme is employed.
The capacitive touch panel that employs the mutual capacitance scheme is configured such that a plurality of drive electrodes to which a detection voltage is applied and a plurality of detection electrodes for detecting a detection voltage level are disposed to be orthogonal to each other; a detection voltage level is monitored at each position of intersection at which a drive electrode and a detection electrode intersects; and the position of an input operation is detected from the position of intersection of the detection electrode at which the detection voltage level is reduced by an input operation object approaching the same (for example, Patent Literature 1).
Such a capacitive touch panel is configured to detect the position of an input operation on the basis of a very small change in the capacitance of the detection electrode, and thus vulnerable to a clock noise occurring on the display device stacked on the front or rear surface or a common mode noise caused by the commercial AC power supply. Thus, these types of noises would lead to an erroneous detection of the input operation position.
As a method for eliminating these noises in order to improve the accuracy of detection of an input operation position, suggested in Patent Literature 2 is a touch panel which extracts an AC signal component as a noise signal from an analog signal that was detected by a sensor in order to detect the input operation position, inverts the resulting signal in phase by 180° and then adds it to the original analog signal, and detects the input operation position from the resulting analog signal with the noise signal cancelled out.
Furthermore, disclosed in Patent Literature 3 is a capacitive touch panel which includes, on the same input operation surface of the touch panel, a main sensor unit for detecting an input operation position and a sub-sensor unit for eliminating noise. The capacitive touch panel disclosed in Patent Literature 3 is configured such that various types of noise signals that occur in the vicinity of the input operation surface are received both by the main sensor unit and the sub-sensor unit, and accordingly, a subtraction unit subtracts the signal received by the sub-sensor unit from the signal received by the main sensor unit so as to eliminate the noises, and then the output from the subtraction unit that includes only a signal provided by an input operation is used to detect the input operation position.